If a mobile station is in an idle state and moves across different Radio Access Technologies (RATs) coverage areas, inter-RAT idle hand-off is conducted. The idle hand-off may include synchronizing the mobile station with the target carrier, measuring/reporting a pilot power of the target carrier, pre-checking in to the serving carrier, and pre-registration upon exiting a border area.
In a conventional system, the idle hand-off is normally triggered by a power measurement and the idle hand-off generally occurs at a sub-net boundary. If there are sharp boundaries between the subnets and a mobile station travels along a sub-net boundary area, a ping-pong issue occurs because the mobile may travel back and forth across the boundary which leads to unnecessary, frequent registrations at both sides of the sub-net. A conventional solution to the ping-pong issue is to use a hysteresis of the power measurement. However, the hysteresis of the power measurement may not be reliable due to shadowing and fading in a mobile environment. The hysteresis of the power measurement is also performed at the expense of power efficiency.
It is also known to introduce additional timer based hysteresis. However, such timer based hysteresis cannot prevent unnecessary idle handoffs if the timer setting is short especially for the access terminals that stay at the coverage border area. In other scenarios, timer-based hysteresis blocks necessary idle handoffs if the timer setting is too long, especially for the mobiles that move at high speed. It is very hard to determine an optimized timer value for mobiles with different speeds.
Another conventional method provides a secondary color code method which uses spatial hysteresis to resolve the ping-pong issue—i.e., base stations located at a border area of a particular subnet transmit both primary and secondary color codes. The primary color code is the color code associated with the subnet in which the base station or cell resides, whereas the secondary color code identifies the bordering subnet. The secondary color code method provides a more efficient method to resolve the ping-pong issue at the sub-net boundary, but only for intra-RAT idle hand-off—i.e., handoffs between systems operating at the same carrier frequency, and the secondary color code method introduces complexity on requesting cross sub-net paging. The secondary color code method only triggers a mobile to register and start the idle hand-off if the mobile exits the border area.
Distance-based hand-off triggering methods are used for both active and idle inter-RAT handoff. However, while such an approach is suitable for triggering active handoff it is too complicated for triggering the idle handoff. It has also been suggested that idle handoff activities be triggered when a mobile station, or access terminal (AT), exits a border area. A problem with this approach is the necessity of a dual mode AT to decode the overhead message from a target RAT (RAT2) when it is attached with a source RAT (RAT1), since the overhead message channel of the source RAT1 is often not reliable at its coverage edge. This approach will also increase the chance, or time gap, of a paging service interruption (i.e., the time gap between determination of a bad radio link with RAT1 (start of handoff) and completion of handoff to RAT2).